1. Field of the Invention
This invention relates to an electron beam device which allows semiconductor wafers of large diameter to be observed with low electron acceleration and high resolution by inclining the wafers at a large angle to the beam.
2. Description of the Prior Art
In the field of semiconductor development and manufacture, scanning electron microscopes (SEMs), which allow wafers of large diameter to be observed with high resolution, while causing them to be inclined at a large angle and applying a low acceleration voltage, have recently been in demand.
In response thereto, an SEM using a conical lens as its objective lens, as shown in FIG. 3, has been developed (Hitachi, Shimizu, et al., Japanese Institute of Electron Microscopes, 29-III-4, 1987). With such an SEM, a resolution of 20 nm has been attained at an angle of inclination of 60.degree. with the application of an acceleration voltage of 1 KV.
However, this resolution is not satisfactory in terms of high resolution observation of semiconductor wafers.
Another type of the SEM providing high resolution is the so-called "in-lens SEM" in which a sample is interposed between the objective lens and a pole piece. Such an SEM has achieved a resolution of 5 nm or smaller at an acceleration voltage of 1 KV. Its resolution is satisfactory but the size small as about 10 mm diameter, thereby making it unsuitable for use in wafer observation.
As an objective lens of the SEM for observing samples of large diameter, it is conceivable to employ a unipolar magnetic field type objective lens. Characteristics of this unipolar magnetic field type lens, including the magnetic field distribution and the aberration coefficient, have been analyzed (Juma, S. M. and Mulve, T., Insti. Phys. Conf. Ser. 52, 1980, pp. 59-60, etc.).
However, the analysis presented in that paper is confined to the type of objective lens in which, as illustrated in FIG. 4, the magnetic pole end surface 11 of an inner tube 10 and the magnetic pole surface 9 of the outer periphery 8 of the lens are in the same plane and the inner tube 10 has no conical surface; moreover that paper discusses no specific structure, including secondary electron detection.
Also, a unipolar magnetic field type lens used as an electron beam tester has been known as well (Japanese patent laid-Open Application No. 69135/1988). This publication discloses, however, no details regarding the shape and dimensions of a unipolar magnetic field objective lens having a small aberration coefficient and allowing a high resolution image to be obtained.
As described previously, further improvements in the degree of integration of VLSIs will unquestionably increase the need to observe wafers of large diameter with a resolution higher than ever at a large angle of inclination and a low acceleration voltage.